1. Field of the Invention
This invention relates to method and apparatus for separating from a stack blank sheets of material supported to retain in a preselected position the stack and feeding the sheets one by one from the bottom of the stack and, more particularly, to apparatus for controlling the separation of each blank from the bottom of the stack to assure that the blank is in proper position to be pulled forwardly from the bottom of the stack.
2. Description of the Prior Art
It is well known in sheet feeding operations to feed sheets of blank material one by one from a stack supported by a rigid member adjacent to the surface of a sheet feeding cylinder. The sheet feeding operation is applicable to a wide variety of sheet-like materials that include photographic film, corrugated blanks for making boxes, folded newspapers, envelope blanks, and the like. The feeder devices are generally classified as top-sheet feeders and bottom-sheet feeders.
In a top-sheet feeder the uppermost or top sheet is removed from a stack by the feeder mechanism and serially conveyed to other devices, for example, a film sheet feeder as disclosed in U.S. Pat. No. 3,265,383. In a bottom-sheet feeder the lowermost sheet is removed from the stack by the feeder mechanism and conveyed serially to other devices. U.S. Pat. Nos. 3,599,970; 3,790,163; and U.S. Pat. No. 3,998,449 disclose feeding envelope blanks from the bottom of a stack in an envelope machine.
As disclosed in U.S. Pat. No. 3,599,970 envelope blanks fed from the bottom of a stack to a conveyor form a band of overlapped envelope blanks. The stack of envelope blanks rests on rotary supporting and separating discs. A feeder removes the blanks from the stack by a sucker shaft which separates the lowermost blank from the stack so that the blank may be removed from the stack and transferred to the surface of a rotating cylinder or to a series of conveyor belts as disclosed in U.S. Pat. Nos. 2,241,474; 2,954,224; 3,141,667; and U.S. Pat. No. 3,160,081. With this type of feeder a feed cylinder is located adjacent to the bottom of the leading edge of the stack in a position to receive the blanks and move them downwardly away from the stack.
In combination with a feed cylinder it is known to mount a suction picker for oscillation through a path clear of the cylinder. The blanks are advanced along the path by the cylinder. One or more deflectors pivotally mounted relative to the front edge of the bottom of the stack move at an angle to the direction of picker travel to push each blank clear of the picker and into conformity with the surface of the cylinder.
To initiate separation of the lowermost blank from the stack the picker is pivoted into position to engage the forward or leading edge of the lowermost blank. Upon application of suction or vacuum to the nozzle of the picker, the lowermost blank leading edge is bent or deflected downwardly away from the stack. The leading edge of the lowermost blank moves downwardly upon downward movement of the picker while the trailing edge of the blank remains fixed. A deflector then moves into position to support the front edge of the stack as the bottom blank continues to bend downwardly and is engaged on the surface of a feed cylinder. Rotation of the feed cylinder carries the bottom blank in a path that removes it from the bottom of the stack.
The known pickers and deflectors for sheet feeding mechanisms are pneumatically and mechanically operable. The picker effects the initial separation of the blank from the bottom of the stack. The deflector is a mechanically movable arm positioned between the initially separated blank and the bottom of the stack to permit the leading edge of the blank to move onto the feed cylinder or into engagement with a series of conveyor belts. The picker and deflector are operated sequentially. When one of them is operating on a blank the other is returned to position for the next operation. This is essential in high speed operations so that when the operation of one blank is commenced the operation of the other blank is still in progress, such as disclosed in the sheet feeding mechanism disclosed in U.S. Pat. No. 1,771,652.
One factor that slows the feeding operation is allowing the picker ample time to exert a vacuum or suction force on the leading edge of the bottom blank so that the leading edge is securely engaged before the picker moves downwardly away from the stack. The actuation of the vacuum force and downward movement of the blank leading edge must be performed rapidly. Then the leading edge must be completely separated from the bottom of the stack so that it is engaged along the entire length of the leading edge by the pressure cylinder. If the separation of the leading edge is incomplete and not positively controlled, then the bottom blank will become jammed between the bottom of the stack and the feed cylinder.
An early approach in paper feeding devices for assuring separation of the forward edge of the bottom blank from a stack before the bottom blank is removed from the stack is disclosed in U.S. Pat. No. 999,936. The stack is supported by counterrotating discs which include cutout portions for exposing during a portion of rotation of the discs the leading edge of the stack. When the cutout portion of the discs is positioned beneath the stack leading edge suction devices are swung into contact with the bottom blank and held there for a period of time to produce sufficient suction to grip the leading edge. When the suction device is swing downwardly the leading edge of the sheet is drawn with it and completely separated from the bottom of the stack.
The suction force upon the leading edge acts for the period of time to permit the forward edge of the bottom blank to be engaged by a feed cylinder. Rotation of the feed cylinder pulls the bottom sheet from the stack. With this arrangement the bottom sheet is not pulled from the stack until the leading edge is completely engaged and positively separated from the bottom of the stack. Thus controlled separation of the leading edge of the bottom sheet is required to prevent jamming in high speed sheet feeders.
Controlled separation of the leading edge of the bottom sheet or blank of a stack is readily accomplished for blanks having a relatively narrow and continuous forward or leading edge, as encountered for example with wallet-size envelopes. For larger size blanks as encountered in making catalog-size envelopes or envelope blanks having a contoured leading edge formed by die cutting separation of the leading edge of the blank is more difficult to control. Many sheet feeders are designed to feed blanks of a fixed dimension; however, sheet feeders have been proposed to accommodate blanks of different sizes. For example, U.S. Pat. Nos. 1,808,706 and 2,799,497 disclose sheet feeders having provision for lateral adjustment to act upon stacks of blanks of different size to assure positive and controlled separation of the bottom blank before it is removed from the stack.
Another approach to providing sheet feeders operable to accommodate a range of blank sizes are feeders having segmented feed cylinders with suction devices positioned along the length of the cylinder between the segments, as disclosed in U.S. Pat. Nos. 1,312,529 and 2,425,936. It has also been proposed by the prior art devices to utilize pressurized air in combination with suction or vacuum to assure complete separation of the bottom blank from the stack to prevent jamming in the feeding operation.
U.S. Pat. No. 1,804,476 discloses feeding blanks from the top of a stack where suction rolls pull the blank upwardly away from the stack. Then a blast of air blows the separated blank against a pair of rotating cylinders which transfer the blank to a conveyor mechanism.
In U.S. Pat. No. 3,380,353 a sheet feeder for envelope blanks includes pivotal blower heads positioned above the feed cylinder opposite the leading edge of the stack. Air blasts are discharged from the blower heads downwardly toward the upper surface of the bottom blank that has been bent downwardly by a sucker head. At the time when the air blast is applied from blower heads, the suction in the sucker head is relieved so that the downwardly diverted air may bend the blank past the sucker head and into position to be engaged by the feed cylinder.
A combination of suction or vacuum force and pressurized air utilized in a sheet feeding apparatus for printing machines and the like is disclosed in U.S. Pat. No. 3,385,593. Air under pressure is directed upon the bottom sheets in a stack to provide a cushion support. This relieves the weight of the stack from the bottom sheets to facilitate separation of the sheets from the bottom of the stack during feeding. The air is supplied externally above the feed cylinder directed downwardly upon the forward edge of the bottom sheets of the stack.
U.S. Pat. No. 3,655,181 discloses a printing press feeder including a nozzle connected to a source of pressurized air directed toward the bottom of the stack. A jet of air is directed between a feed drum and a transfer roller opposite the front edge of the bottom of the stack to assure that only one piece of paper at a time is transferred to the feed drum from the bottom of the stack.
In U.S. Pat. No. 3,674,255 an air blast is used in a sheet feeding device to facilitate separating the trailing edge of a sheet from the feed or suction roller after the sheet has been removed from the bottom of a stack and transferred to the suction roller. This arrangement is intended to prevent tearing of the sheet as it is conveyed by the suction roller from the stack.
U.S. Pat. No. 3,405,935 discloses a mechanism for separating IBM punch cards from one another in a stacked arrangement by feeding the cards from the bottom of a stack on a vacuum belt. To prevent the two cards from being conveyed at once because the vacuum acting on the lowermost card can be transmitted through the punched holes to the card above, the lowermost card is subject to atmospheric air. With this arrangement an air cushion is created between the lowermost and second lowermost cards.
In U.S. Pat. No. 4,052,050 labels are removed from the bottom of a stack by a rotating picker in which the leading edge of the label is engaged by a vacuum created on the surface of the picker and then is transferred onto the surface of a feed roll with the assistance of a jet of air supplied through a tube positioned above and externally of the picker. U.S. Pat. Nos. 3,861,667; 4,194,442; 4,320,893; 4,542,894; 4,681,311; 5,028,043; and U.S. Pat. No. 5,028,044 are further examples of more recent developments in sheet feeding operations where blanks are fed at high speed one by one from the bottom of a stack.
While a number of prior art devices have proposed improvements to sheet feeding mechanisms to assure positive separation of the bottom sheet from a stack in high speed feeding operations, the prior art devices have not provided an acceptable solution to high speed feeding of stacked blanks having an extended leading edge with an irregular contour. With the prior art devices described above the leading edge of the blank is relatively narrow, particularly for blanks used to make wallet-size envelopes or blanks having a diamond shape configuration and relatively narrow in width. The known devices satisfactorily initiate separation of the bottom blank from the stack where the width of the blank corresponds in length to the length of the sucker shaft or even in a larger blank having a leading edge which is free of any contour or die cut irregularity. This is not the case for large blanks, such as those used to make catalog-size envelopes or envelopes having a center seam or a side seam where a portion of the leading edge is die cut. With a blank having a die cut leading edge the intermediate portion thereof extends or leads the lateral portions of the leading edge. In other words, the lateral portions of the leading edge are recessed or displaced rearwardly from the intermediate portion of the leading edge.
The problems encountered in feeding from the bottom of a stack blanks having die cut leading edges is that the intermediate portion is initially engaged by the sucker shaft of the feed cylinder disclosed in U.S. Pat. No. 3,599,970, and the lateral portions of the leading edge lag behind. For large size blanks used in making center seam or side seam envelopes the recessed lateral portions can have a length almost as great as the leading edge intermediate portion. Consequentially the lateral portions of the leading edge do not follow the intermediate portion when the intermediate portion is first engaged by the sucker shaft. The lateral portions being positioned rearwardly of the leading edge are not subject to the suction force acting on the intermediate portion.
The leading edge intermediate portion is deflected downwardly and effectively separated from the bottom of the stack, but the leading edge lateral portions are uncontrolled and may still remain in contact with the bottom of the stack when the feed cylinder begins to remove the bottom blank from the stack. As the blank is bent downwardly by the sucker bar and engaged by the feed cylinder the remaining portions of the blank, which have yet to be engaged by the sucker bar, follow a different feed path which may result in jamming of the bottom sheet, especially as operating speed increases.
Therefore there is need in high speed sheet feeding of large size blank material and particularly large center seam and side seam envelope blanks for a mechanism to positively control movement of the contoured lateral front edge of the blank to assure that the leading edge is evenly separated from the bottom of the stack before the blank is fed from the stack.